INSENSITIVITY OF THE HIPPOCAMPUS TO ENVIRONMENTAL STIMULATION DURING POSTNATAL-DEVELOPMENT

Development of cortical sensory systems is influenced by environmental experience during ''sensitive periods,'' before onset of behavioral f unction. During these periods, synaptic plasticity is observed, and ne uronal function shows increased responsiveness to environmental stimul ation. Because the hippocampus is late to develop, and because it demo nstrates synaptic plasticity before the onset of behavioral function, this experiment was designed to determine whether, like the sensory co rtices, the hippocampus undergoes a period of enhanced responsiveness to the environment. Rats at three ages [postnatal day 16 (P16), P23, a nd P30] were tested on a hippocampally dependent task, spontaneous alt ernation, and exposed to a novel environment. They were then killed an d processed for immunocytochemistry to Fos or for in vitro electrophys iology in hippocampal area CA1. Age-matched control subjects were kill ed immediately after removal from the home cage. Spontaneous alternati on was only observed in the oldest (P30) animals. In these same animal s, the environmental manipulation resulted in an increase in Fos-like immunoreactivity (FL-IR), relative to controls, and a decrease in the ability to induce long-term potentiation (LTP). In P16 and P23 animals , the environmental manipulation resulted in no differences in hippoca mpal FL-IR or LTP. These results suggest that, rather than showing inc reased responsiveness to the environment at these ages, the hippocampu s is environmentally insensitive and that it is isolated from the effe cts of environmental stimuli. The hippocampus, a neural region importa nt for higher cognitive function, may develop via a mechanism differen t from those observed in the primary sensory cortices.